This invention relates generally to imaging systems capable of scanning objects in multi modalities and more particularly to a method for determining an alignment status of the multi-modality system.
The present invention is directed toward multi-modal imaging systems capable of scanning using different modalities, such as, for example, but not limited to, Positron Emission Tomography (PET) and Computed Tomography (CT). The difference between multi-mode and multi-modality is that in multi-mode systems the same hardware is utilized to perform scans in different modes (e.g., a radiation source and a radiation detector is used in both a flouro mode and a tomosynthesis mode), while in a multi-modal system (multi-modality system), although some of the same hardware is utilized to perform different scans (e.g., an image produced by PET is processed and displayed respectively, by the same computer and display, as an image produced by CT), the data acquisition systems (hereinafter sometimes termed “modality unit”) are different. For example, on a CT/PET system, a radiation source and a radiation detector are used in tandem to acquire CT data, while a radiopharmaceutical is typically employed in tandem with a PET camera to acquire PET data. It is contemplated that the benefits of the invention accrue to all multi-modality imaging systems, such as, for example, but not limited to, a CT/PET imaging system as well as systems utilizing currently unknown modalities as well as currently unfeasible combinations, such as, for example, but not limited to, a combination PET/ultrasound system and/or a CT/MRI system.
In such multi-modality systems, such as, for example, an integrated PET-CT system there is an inherent registration of the PET and CT images the system acquires. Since the patient lies still on the same table during the PET and CT portions of the acquisition, the patient will be in a consistent position and orientation during the two acquisitions, greatly simplifying the process of correlating and fusing the CT and PET images. This allows the CT image to be used to provide attenuation correction information for the reconstruction of the PET image, and allows an image reader to easily correlate the anatomic information presented in the CT image and the functional information presented in the PET image.
This inherent registration assumes a perfect alignment of the PET and CT detector coordinate systems, or at least a known spatial transformation between the two coordinate systems. This should be readily apparent, since a misalignment of the coordinate systems will directly result in a misregistration of the images.
Proper PET and CT image registration also requires an alignment of the axial (z-) axis of the PET and CT coordinate systems not only with each other and with the travel axis of the table that transports the patient during the PET and CT acquisitions. This may be less obvious, but it is a consequence of the differing nature of the PET and CT acquisitions. A volume of CT image data is acquired slice-by-slice, or, in the case of a multislice helical scan, by a detector system with a small axial extent of a few centimeters. By contrast, a PET scan is acquired in a number of frames of data, each of which may cover an axial extent of 15 cm or more. Even if the CT and PET are perfectly aligned, a misaligned table will produce different artifacts in the CT and PET volumes, which cannot be properly registered. Accordingly it is desirable to provide a method for determining an alignment status of the multi-modality system.